The major focus of our research effort has centered on the molecular mechanisms which regulate protein synthesis in reticlocyte lysates. This is a highly efficient in vitro laboratory model for studies on the synthesis of globin due to (a) the rapid and prolonged linear rate of globin synthesis in vitro, (b) the complete dependence of globin synthesis on exogenous hemin, and (c) the absence of transcriptional events. In the absence of added hemin, protein synthesis proceeds at optimal linear rates for several minutes and then ceases abruptly. Inhibition is due to the activation of a translational inhibitor whose formation is regulated by heme. Addition of the isolated inhibitor to heme-supplemented lysates induces an inhibition similar to that observed in heme-deficiency. In the past year, considerable progress has been made due primarily to two fundamental achievements: (1) The translational inhibitor formed in heme-deficiency has been purified to near homogeneity. (2) The purified inhibitor has been identified as a cyclic AMP-independent protein kinase which specifically phosphorylates the small polypeptides component (38,000 daltons) of the protein chain initiation factor (IF-MP), which binds Met-tRNAf in a ternary complex; the finding that IF-MP reverses both of the inhibitions of protein synthesis induced by heme-deficiency or the purified inhibitor suggests that this factor is the target of the action of the inhibitor. On the basis of this and other evidence, we propose a model for inhibition which incorporates the following features: (a) Inhibitor formation begins early in the initial linear phase of protein synthesis in heme-deficient lysates. (b) In the presence of active inhibitor, IF-MP is utilized stoichiometrically rather than catalytically. (c) When the pool of function IF-MP is depleted, protein synthesis shuts off. (d) Addition of exogenous IF-MP to inhibited lysates restores synthesis in proportion to the amount of added IF-MP. (e) A phosphorylation-dephosphorylation mechanism may occur normally in the re-cycling of IF-MP; the inhibitor interferes with this equilibrium. The further finding that similar translational controls exist in non-erythroid tissues such as rat liver and Ehrlich ascites tumor cells suggests that these regulatory mechanisms may be widespread.